1. Field of the Invention
The present invention relates to a color conversion definition creating method of creating a color conversion definition for converting first color data representative of coordinate points within a first color space depending on an input device for obtaining image data through entering an image into second color data representative of coordinate points within a second color space independent of a device, and a color chart for creating the color conversion definition, in which a plurality of mutually different colors of color patches are arranged.
2. Description of the Related Art
Colors of lights emitted from the respective points of an image are provided as data in form of coordinate values of three-dimensional color space, such as RGB (R: red, G: green, B: blue), CMY (C: cyan, M: magenta, Y: yellow), XYZ, and L*a*b* , in connection with the fact that human eyes have three sorts of cell sensitive to three colors of red, green and blue. For example, a scanner reads lights emitted from the respective points of an image through filters of red, green and blue to create output data (R, G, B) represented by the coordinate values of the color space RGB.
The output data (R, G, B) depends on spectral distribution characteristics of the above-mentioned filters and a photo detector unit of the scanner. And thus the output data (R, G, B) is data having a device dependency inherent in the scanner. Because of the device dependency, the output data of the scanner is inadequate for using in another image apparatus such as a printer per se. However, in the event that there is prepared a color conversion definition defining the association between the output data (R, G, B) of the scanner and color data free from device dependency such as calorimetric value data (L*, a*, b*) represented by coordinate values of a color space L*a*b*, and in addition, also for another image apparatus, there is prepared a color conversion definition defining the association between color data treated by the another image apparatus and calorimetric value data, a combination of those color conversion definitions makes it possible to convert the output data (R, G, B) of the scanner into the color data of a color space involved in device dependency of another image apparatus through calorimetric value data (L*, a*, b*) free from device dependency.
The association between the output data of the scanner and the calorimetric value data can be created as follows. First, a laser printer is used to perform an exposure on for example a reversal film (RV), so that a color chart having a plurality of patches is created. The scanner reads transmitted lights through the patches of the color chart so as to create output data (R, G, B) of the scanner associated with the patches. And the transmitted lights through the patches of the color chart are measured so as to create calorimetric value data (L*, a*, b*) associated with the patches. In this manner, the patches are used to associate the created output data (R, G, B) of the scanner with the created calorimetric value data (L*, a*, b*), and this correspondence is expressed on an approximate basis using for example a polynomial expression to create a color conversion definition for converting the general output data (R, G, B) into calorimetric value data (L*, a*, b*).
Usually, an area defined by a plurality of color patches of a color chart, which area is referred to as a color reproduction area (gamut) hereinafter, does not include an entire area on which the reversal film (RV) is able to inherently form a color, and a dynamic range of a scanner is broader than the area on which the reversal film (RV) is able to inherently form a color. For this reason, the color conversion definition at the exterior of the color reproduction area of the color chart is created in such a manner that for example, the polynomial expression, which is used for the association between the calorimetric value data (L*, a*, b*) and the output data (R, G, B) of the scanner, is simply expanded to the exterior of the finite area.
The color conversion definition is created in the manner as mentioned above. However, according to the former color chart such as IT8 chart used for creating such a color conversion definition, it is difficult to create the color conversion definition with great accuracy, and particularly it is difficult to create the color conversion definition with great accuracy in the vicinity of the boundary of the color reproduction area of the color chart and at the exterior of the color reproduction area of the color chart.